1. Field of the Invention
The present invention is related to a microwave oven with a function of induction heating and the control method thereof, and particularly to a microwave oven incorporating an induction coil, which heats the foods to be cooked by a heating manner suitable for the cooking vessel after sensing the material of the cooking vessel.
2. Description of the Prior Art
For the purpose of improving the taste of foods, an additional heating instrument has been incorporated in the microwave oven which cooks foods utilizing the principle of dielectric heating. For example, one type of known microwave oven includes an electric heater in the heating chamber thereof, in which the electric heater is alternately or concurrently activated with the dielectric heating unit to increase the taste of foods.
In addition, another known type of microwave oven includes an induction coil which produces a high frequency magnetic field to cause eddy currents to flow through a cooking vessel made of steel or stainless steel, and thereby heats the foods by the Joule heat produced in the cooking vessel. A typical microwave oven with induction coil is disclosed in Japanese patent laid-open No. 91.107016. In the disclosed microwave oven, the induction coil is arranged between a wave guide mounted on the heating chamber and a outer case. A magnetic shielding member is provided for preventing the wave guide from being heated by the magnetic field produced by the induction coil, for example, an aluminum sheet mounted on the wave guide, and a thermal insulation member normally made of a mica sheet mounted on the outer case.
However, the disclosed microwave oven simply incorporates the induction coil, without any functionally combined relationship. Accordingly, dielectric heating or induction heating cannot be performed without both moving the cooking vessel and/or moving foods from one vessel to another suitable for the heating manner.
To alleviate the aforementioned problem, another microwave oven has been developed with an induction coil being arranged in the space formed between the bottom wall and the base cover as disclosed in U.S. Pat. No. 5,177,333.
The disclosed microwave oven comprises a heating chamber whose bottom wall is made of a non-magnetic metal mesh having a size in a range from 10 mesh to 25 mesh, an induction coil disposed in the vicinity of the outer bottom wall of the heating chamber, and a turn table preferably made of dielectric material. In the disclosed microwave oven, if the user selects a specific function key, (for example, a "frying pan" key), the microwave oven initially performs induction heating for a predetermined time interval to cook the protein in the surface portion of the meat, and then automatically performs dielectric heating to heat the internal portion of the meat.
With regard to this last point, the microwave oven disclosed in the latter patent publication has the advantage in that the user doesn't have to move the foods from one vessel to another suitable for the subsequent heating manner, or move the vessel itself because it is functionally combined with the induction heating part thereof to some extent, as compared to the microwave oven disclosed in the former patent publication.
However, when the "frying pan" function is performed in the state that the foods are contained in a steel vessel with a tall body part, the microwave oven disclosed in the latter patent publication still has a problem in that the foods are not satisfactorily cooked because the amount of the microwave energy reflecting from the body part of the cooking vessel increases during dielectric heating. In other words, the cooking performance is subjected to restrictions depending on the shape or material of the cooking vessel. Furthermore, the microwave oven does not automatically execute cooking using the heating manner suitable for the material of the cooking vessel.